Electronic circuits typically employ hard-wired communication links to convey information within circuit boards as well as between and among local and remote circuit boards. One such type of communication link (e.g., a pair of traces such as interconnects 116-1 and 116-2) is shown in communication system 100 (e.g., a differential mode transmitter-receiver communication system) as illustrated in FIG. 1. As shown, communication system 100 includes differential mode transmitter 110-1 and differential mode transmitter 110-2 (collectively transmitters 110) that transmit corresponding electronic signals X1=X, X2=−X, X5=Y, and X6=−Y on respective interconnects 116-1, 116-2, 115-1, and 115-2 to differential mode receivers 120-1 and 120-2. Electronic signals X1, X2, X5, and X6 are generated by differential mode transmitters 110 based on digital input signals 105-1 and 105-2. Differential mode receivers 120-1 and 120-2 generate respective digital output signals 125-1 and 125-2 based on electronic signals X3, X4, X7 and X8 received at corresponding inputs.
During differential mode operation, differential mode transmitters 110 generate balanced output signals of opposite polarity. For example, when input signal 105-1 is a logic low, differential mode transmitter 110-1 produces a 0.5 volt output at node 1 of differential mode transmitter 110-1 and a 1.5 volt output at node 2 of differential mode transmitter 110-1. Conversely, when input signal 105-1 is a logic high, differential mode transmitter 110-1 produces a 1.5 volt output at node 1 and a 0.5 volt output at node 2. Signals X and −X are thus conveyed over respective traces 116-1 and 116-2 to differential mode receiver 120-1 that converts the received signals at corresponding input nodes 3 and 4 into output signal 125-1. When differential mode receiver 120-1 detects a difference signal greater than the negative noise margin, i.e., −(X3−X4)>NMN, where NMN is a positive number, e.g., 0.1 V, receiver 120-1 produces a logic low at output signal 120-1. Conversely, when differential mode receiver 120-1 detects a difference signal greater than the positive noise margin, i.e., X3−X4>NMP, where NMP is a positive number, e.g., 0.09V, differential mode receiver 120-1 produces a logic high at output signal 120-1. In a similar way, transmitter 110-2 simultaneously transmits signals Y and −Y to convey information over interconnects 115-1 and 115-2 to receiver 120-2.
High frequency signals sent across interconnects 115 and 116 are modified by the conducting paths. Among other reasons, this may due to dielectric losses in the insulating material surrounding the conductive paths, due to skin effect losses in the conductors themselves, due to impedance variations along the conductive paths. In the absence of coupling between paths 115 and 116, the signals received at nodes 3 and 4 can be described as 1X3=H31X1+H32X2, and X4=H41X1+H42X2, respectively, where H31 and H32 are the transfer functions that capture the effect of the path 116 with respect to receiving node 3, and H41 and H42 capture the effect of path 116 with respect to receiving node 4. This representation implies a frequency domain representation of node signals, and transfer functions.
Conveying balanced and oppositely polarized signals (i.e., differential mode communication signals) such as X and −X in this way generally enables higher speed communication because of reduced susceptibility to noise (e.g., cross-talk). However, when pushing the limits on a particular link (e.g., a pair of traces such as interconnects 116-1 and 116-2) such as transmitting in the Gigahertz range, the signals X3, X4, X7, and X8 may become corrupt due to effects such as intersymbolinterference (ISI) and cross-talk. For example, because interconnect 116-1 is parallel and in close proximity to interconnect 115-1, a certain amount of cross-talk Yc couples from signal X5 onto interconnect 116-1; the voltage at node 3 of receiver 120-1 is X3=H31X1+H32X2+Yc. Based on coupling of cross-talk −Yc caused by signal −Y onto interconnect 116-2, the voltage at node 4 of differential mode receiver 120-1 is X4=H41X1+H42X2−Yc. The effects of cross-talk Yc and −Yc may be so great as to limit the rate at which differential mode transmitter 110-1 effectively transmits data to differential mode receiver 120-1. In a similar way, the effects of cross-talk from signal X1, X2 onto respective interconnects 115-1 and 115-2 may be so great as to limit the rate at which differential mode transmitter 110-1 effectively transmits data to differential mode receiver 120-2.
The above example illustrates a straight-forward technique of supporting differential communications via use of analog circuitry and the ill-effects of cross-talk. Conventional methods such as those based on DSP (Digital Signal Processing) may employ techniques to mitigate the effect of cross-talk. In general, DSP involves converting an analog signal to a corresponding digital signal which, in turn, is processed by a high speed processor device to recover an original signal encoded with information.
One method of addressing cross-talk is to learn the behavior of an interfering channel (e.g., a particular link) by sending known symbols from the aggressor transmitter and analyzing the response at the victim receiver. Once this behavior is learned, the interference signal under normal operation can be estimated and therefore canceled. Thus, in general, this method involves attempting to actively cancel cross-talk based on the learned information.
Often linear equalizers are used to correct for frequency dependent loss—which causes inter-symbol-interference (ISI)—in the channel. Unfortunately, this technique also enhances interference at high frequencies. Another method uses Decision Feedback Equalization (DFE); here a scaled form of the previous decisions is used to correct for Inter-Symbol-Interference in the current symbol. In this way, DFE does not enhance cross-talk while it corrects for ISI. In general, this method is effective in discerning the desired signal without necessarily exacerbating the effect of interference.
Yet another method of signal estimation in the presence of cross-talk, in contrast to determining each symbol individually in time, is to employ Maximum Likelihood Sequence Estimation (MLSE) that involves picking the most likely sequence of symbols to make a decision.